Conventionally, electric storage devices that are repeatedly useable, such as secondary batteries, have been used. The field of use of the electric storage devices including electric vehicles is currently expanding.
Conventionally, a technology for estimating a deterioration level of a secondary battery using an open circuit voltage characteristic of the secondary battery has been known (e.g., JP-A-2011-220917). In the secondary battery, information on a correlation between electrode potentials (positive electrode potential and negative electrode potential) and a battery capacity varies according to deterioration. In the conventional technology, the open circuit voltage characteristic that represents information on a correlation between the open circuit voltage and the internal condition of the secondary battery is measured. Then, parameters including a capacity retention rate of the positive electrode, a capacity retention rate of the negative electrode, and a variation in battery capacity are estimated such that the parameters correspond to the measured open circuit voltage characteristic. In the conventional technology, the parameters are estimated and the information on a correlation between the electrode potential of the secondary battery and the battery capacity thereof is defined. Then, the deterioration level of the secondary battery is estimated.
Unlike the conventional technology, a deterioration level of an electric storage device such as a secondary battery can be estimated based on the open circuit voltage characteristic of the electric storage device, which is estimated by defining information on a correlation between an electrode potential of the electric storage device and an electric storage capacity thereof. In this case, a process for defining the information may be complicated if the conventional technology is used to define the information because the conventional technology includes multiple processes for individually estimating multiple parameters. Therefore, the open circuit voltage characteristic of the electric storage device cannot be easily estimated.